The terms "electrography" and "electrographic" as used herein broadly include various processes that involve forming and developing electrostatic charge patterns on surfaces, with or without the use of light. They include electrophotography and other processes. One method of electrographic development is the magnetic brush method which is widely used for dry development in electrophotographic document copying machines. It is disclosed, for example, in U.S. Pat. No. 3,003,462. Such a developer is a mixture of thermoplastic toner particles and magnetic carrier particles, the latter of which may optionally be coated with an insulating resin in order to enhance the tribocharging capability of the carrier particles.
In the development station of a copying machine, the two-component developer, which includes the magnetic carrier particles, is applied to the electrostatic charge pattern by means of a magnetic applicator which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternative north/south magnetic fields. These fields project radially, through the sleeve and serve to attract the developer composition to the sleeve's outer surface to form a brush nap. Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position close to or in contact with the electrostatic charge pattern to be developed, e.g., as in the patent to Miskinis et al., U.S. Pat. No. 4,546,060. By frictional contact with the carrier particles, the toner particles are triboelectrically charged and cling to the carrier particles, creating bristle-like formations of developer on the magnetic brush sleeve. In developing a charge pattern, the brush is brought close to or in contact with the charged surface. The oppositely charged toner particles are drawn away from the carrier particles on the magnetic brush by the more strongly charged electrostatic charge pattern, thus developing and making visible the charge pattern.
Especially useful as the carrier particles in two-component developers are strontium and barium ferrites. Ferrites, as used herein, are magnetic oxides containing iron as a major metallic component. The ferrites of strontium and barium referred to herein are the ferrites of strontium and barium, having the formula SrFe.sub.12 O.sub.19 and BaFe.sub.12 O.sub.19. These ferrite carriers are disclosed in U.S. Pat. No. 4,546,060 to Miskinis et al and U.S. Pat. No. 4,764,445 to Saha, both of which are incorporated herein by reference. Strontium and barium ferrites, being hard magnetic materials, are desirable as carrier particles. The use of such "hard" magnetic materials which exhibit a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g when in an applied magnetic field of 1000 Oersteds as carrier particles has been found to dramatically increase the speed of development when compared to conventional magnetic carriers made of relatively "soft" magnetic materials such as magnetite, pure iron, ferrite or a form of Fe.sub.3 O.sub.4 having magnetic coercivities of about 100 gauss or less.
The terms "hard and "soft" when referring to magnetic materials have the generally accepted meaning as identified on page 18 of Introduction to Magnetic Materials by B. D. Cullity, published by Addison-Wesley Publishing Company, 1972.
As such, these hard magnetic carrier materials represent a significant advancement in the art over the previously used soft magnetic carrier materials in that the speed of development is remarkably increased. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
However, a problem that has been encountered with such hard magnetic ferrite carrier particles containing strontium and barium is that these materials have not always been found to be satisfactory with respect to copy image density. That is, it has been observed that as development speed or efficiency progressively increases using developer compositions comprising such hard ferrite carrier particles and oppositely charged toner particles, the density of the developed images produced thereby progressively decreases. This is particularly noticeable in the solid, colored image area portions of the toner image which appear lighter or fainter in appearance than desired. This is due primarily to the progressive inability of the carrier particles to deliver an amount of toner particles sufficient enough to completely develop the electrostatic charge pattern on the charged surface as the development speed progressively increases. While not wishing to be bound by any theory, it is believed that this phenomena is due to the following.
In the development of electrostatic charge patterns utilizing a rotating-core magnetic applicator described above, the hard magnetic ferrite carrier particles join together to form hair-like chains or bristles of carrier particles which extend outward from the sleeve or outer shell of the rotating-core magnetic applicator. As the core rotates, the field from each pole of the magnets within the applicator travels circumferentially around the outer surface of the shell. As a result, these chains of carrier particles are exposed to a succession of magnetic fields emanating from the rotating-core applicator and are caused to flip or turn to move into magnetic alignment in each new field. Each flip is accompanied by a rapid circumferential step by each particle in a direction opposite the movement of the rotating core. The observed result is that the developer flows smoothly and at a rapid rate around the shell while the core rotates in the opposite direction thus delivering fresh toner to the electrostatic charge pattern. However, the length of the carrier particle chains which are formed on the outer surface of the sleeve or shell of the rotating-core magnetic applicator are not long enough to provide a carrier surface area large enough to triboelectrically charge an amount of toner particles sufficient enough and at a charging level high enough to completely develop the electrostatic charge pattern to be developed as the development speed progressively increases.
We have now found, however, that this problem can be obviated by using, as a toner carrier, an electrostatographic ferrite carrier which comprises hard magnetic ferrite material having a single phase, W-type hexagonal crystalline structure represented by the general formula MFe.sub.16 Me.sub.2 O.sub.19 where M is strontium or barium and Me is a divalent transition metal selected from the group consisting of nickel, cobalt, copper, zinc, manganese, iron and mixtures thereof, and exhibits a coercivity of from approximately 100 Oersteds to 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 60 EMU/g when in an applied magnetic field of 1000 Oersteds. We have found that copy image density utilizing the ferrite carriers of the present invention can be increased by at least three times over the copy image density obtained with the hard ferrite carriers of the prior art previously described at equivalent high development speeds. How this is accomplished is described in detail below.